Method and device for measuring intermodulation distortion

ABSTRACT

A measuring device is provided for measuring intermodulation distortion of a measuring object. The measuring device includes a first signal generator which produces a first signal that is supplied to an input of a measuring object, a signal combining device having a first input which is connected to the output of the measuring object, and a signal analyzing device which is connected to the output of the signal combining device. According to the disclosure, a second signal generator which is synchronized with the first signal generator is provided, and second signal generator producing a second signal that is supplied to a second input of the signal combining device.

The invention relates to a method and a device for measuring intermodulation distortions.

Intermodulation distortions (d3-intermodulation), especially in the form of adjacent-channel power (ACPR) represent an important specification especially for power amplifiers, which are used in mobile telephone base stations. This also applies in particular to power amplifiers, which are driven with several carrier signals. Nonlinearities in the power amplifier lead to crosstalk from one channel into the adjacent channels, causing an interference effect. To measure the intermodulation distortions of a power amplifier, the amplifier is therefore supplied with a measuring signal in the useful channel, and the power components in the adjacent channels are measured.

A method and a device for measuring intermodulation distortions in a-device under test, such as a power amplifier according to the preamble of claim 1 and claim 9, are already known from document U.S. Pat. No. 6,263,289 B1. With the method according to this document, a signal generator is connected to the input of the device under test (DUT). The device under test may, for example, be a power amplifier, which is terminated with a terminal resistance. A directional coupler is provided at each input and output of the device under test. The signal of the coupler at the output of the device under test is supplied directly to a signal combiner (power combiner), while the signal picked up via the coupler at the input of the device under test is supplied to the signal combiner through a network for modifying the level and the phase. The network for modifying the level and phase subjects the signal picked up at the input of the device under test to the same level and phase changes experienced by the signal in the device under test, but this signal pathway is subjected to an additional phase displacement of 180° relative to the signal pathway leading through the device under test. When measuring the adjacent-channel power (ACPR), adjacent-channel power components which are already contained in the input signal of the device under test are therefore compensated.

The method known from U.S. Pat. No. 6,263,289 B1 has the disadvantage that a delay compensation is possible only within the range of a few 100 ns, because a larger delay compensation cannot be achieved by the network for modifying level and phase. Devices under test with longer delays cannot therefore be measured using the known methods. It is also disadvantageous that a network for modifying the level and phase, which operates with sufficient accuracy, is structured in a relatively expensive manner, as shown in FIG. 6 of U.S. Pat. No. 6,263,289 B1. The known method is therefore associated with a relatively high realisation costs.

The present invention is based on the object of providing a method and a measuring device for measuring intermodulation distortions in a device under test, which allows measurements to be made with increased accuracy but reduced costs.

This object is achieved with reference to the method by the features of claim 1 and with reference to the measuring device by the features of claim 9.

The invention is based on the knowledge that a considerably greater measuring accuracy can be achieved at reduced cost by using separate signal generators for the measured signal and the reference signal. With the solution according to the invention, a first signal generator generates the input signal supplied to the device under test, while a second signal generator synchronised with the first signal generator generates the reference signal, which is supplied to the signal combiner. The group delay in the, device under test and also the phase displacement in the device under test can be compensated within a relatively large framework by presenting a corresponding time delay and/or phase displacement of the reference signal generated by the second signal generator relative to the measured signal generated by the first signal generator. Delays through the device under test in the microsecond range, which currently arise in power amplifiers with digital feed-forward linearisation, can also be compensated using the measuring method according to the present invention. With the level-phase matching network defined in the prior art, this is possible only with great difficulty or not at all. Moreover, delay compensation within the band width of the measured signal can take place in a frequency-independent manner, that is to say, over a broad band. With the method according to the invention, a suppression of undesirable signal components in the measured signal of more than 26 dB is achieved. The dependent claims specify advantageous further developments of the invention.

At least one of the signal generators can be adjusted with reference to the time delay, amplitude and phase position both of the high-frequency signal and also of the modulation signal, with which the high-frequency signal is modulated. The phase of the high-frequency signal and also the modulation signal are both adjusted with a 180° phase difference relative to the output signal of the device under test, so that a maximum elimination of the signal occurs. With the level-phase matching networks known from the prior art, phase compensation of the modulation signal separately from the high-frequency signal is never possible, and this therefore represents a further major advantage of the method according to the invention.

Adjustment of the phase position of the high-frequency signal and the modulation signal should preferably be possible with a very small step width preferably less than {fraction (1/50)} of the period, by further preference less than {fraction (1/100)} of the period, so that the 180°-phase compensation can be adjusted extremely accurately. The output signal of the device under test can be linked either via a damping element, of which the damping factor is dimensioned in such a manner that the signal level at the output of the damping element approximately corresponds to the signal level at the input of the device under test, or via a coupler disposed at the output of the device under test, wherein, in this case, the coupling factor should be dimensioned in such a manner that the signal level at the output of the coupler approximately corresponds with the signal level at the input of the device under test.

Exemplary embodiments of the invention are explained in greater detail below with reference to the drawings. The drawings are as follows:

FIG. 1 shows a block circuit diagram of a first exemplary embodiment of a measuring device according to the invention;

FIG. 2 shows the spectrum at the output of the device under test by way of explanation of the problem upon which the invention is based and

FIG. 3 shows a second exemplary embodiment of the measuring device according to the invention.

The exemplary embodiment of the measuring device 1 according to the invention shown in FIG. 1 comprises a first signal generator 2, which generates a first high-frequency signal S₁, which is supplied to the input 4 of a device under test 3 or DUT. In the exemplary embodiment, the output 5 with the output signal S_(A) from the device under test 3 is connected via a damping element 6 to a first input 7 of a signal combiner 8. A second signal generator 10 synchronised with the first signal generator 2 via a synchronisation line 9 is connected to a second input 11 of the signal combiner 8. The signal combiner 8 combines the input signals at the inputs 7 and 11 to form a combined signal, which is supplied to a signal analyser 12. The signal analyser may, for example, be a spectrum analyser or another appropriate measuring device.

The high-frequency signal S₁ generated by the first signal generator 2 and the high-frequency signal S₂ generated by the second signal generator 10 are modulated in the signal generators 2 and 10 with an appropriate modulation signal, so that, for example, a WCDMA (Wide Band Code Division Multiple Access) signal according to a standard of the third generation of mobile telephones (such as the 3 GPP standard) is generated as a test signal. The device under test 3 can be any 2-port device. Power amplifiers are measured by preference. Such power amplifiers are designed to be relatively broadband, so that a high amplification is provided in the useful channel with the minimum possible crosstalk in the adjacent channels. Intermodulation distortions in the form of adjacent-channel power ACPR (Adjacent Channel Power Ratio) must be kept to the minimum. The intermodulation distortions, especially in the form of adjacent-channel power ACPR are measured by the measuring device according to the invention as a specification of the power amplifier to be measured.

By way of explanation, FIG. 2 shows the typical output spectrum of a power amplifier which is to be measured. The diagram shows the level A of the output signal S_(A) of the power amplifier as a function f driven only in the useful channel CH₀. It is evident that, because of non linearities, the power amplifier also generates considerably weakened spectra in the directly adjacent channels CH⁻¹ and CH₁ and in the more remote adjacent channels CH⁻² and CH₂. To measure the adjacent channel power ACPR in the adjacent channels CH⁻², CH⁻¹, CH₁, CH₂ with a high level of accuracy, the power amplifier, and/or, in general, the device under test 3 should only be driven in the useful channel CH₀. However, a real signal generator 2 also generates slight adjacent-channel power components in the adjacent channels, which are already supplied to the inputs 4 of the power amplifier serving as the device under test 3, and are also amplified by this device because of the broad-band design. This falsifies the measurement of the adjacent-channel power ACPR. It is therefore necessary to compensate the adjacent-channel power components generated by the signal generator 2.

For this purpose, the present invention proposes the use of the second signal generator 10, which is preferably of identical structure to the first signal generator 2 and synchronised with the first signal generator 2. Accordingly, the same adjacent-channel power components occur in the high-frequency signal S₂ generated by the second signal generator 10 as in the high-frequency signal S₁ generated by the first signal generator 2. The damping element 6 is dimensioned in such a manner that its damping factor approximately matches the amplification factor of the power amplifier serving as the device under test 3, so that approximately equal signal levels are provided at the inputs 7 and 11 of the signal combiner 8.

The first signal generator 2 and/or the second signal generator 10 comprise devices for adjusting the time delay, the amplitude and the phase position of the signal generated by the signal generator 2 and 10 respectively. In the exemplary embodiment illustrated, a device 13 is provided in the second signal generator 10, for adjusting the time delay ΔT, with which the high-frequency signal S₂ from the second signal generator 10 is emitted, and an adjustment device 14 is provided for adjusting the amplitude A, with which a high-frequency signal S₂ from the second signal generator 10 is emitted. Furthermore, the phase position Δφ, with which the high-frequency signal S₂ is emitted, can be adjusted with an adjustment, device 15. The high-frequency signal is preferably modulated with a modulation signal, for example, in a I/Q modulator. The phase position Δφ_(m) of the modulation signal is preferably adjustable by means of a further adjustment device 16.

The amplitude A, with which the second high-frequency signal S₂ is emitted, is adjusted in such a manner that the amplitudes A₁ and A₂, with which the high-frequency signals S₁ and S₂ of the signal generators 2 and 10 arrive at the signal combiner 8, agree as accurately as possible. The time delay ΔT, with which the second high-frequency signal S₂ is emitted, is adjusted in such a manner that the high-frequency signal S₂ from the second signal generator 10 is delayed relative to the high-frequency signal S₁ from the first signal generator 2 by a time offset, which corresponds as accurately as possible to the group delay of the high-frequency signal S₁ through the device under test 3. Furthermore, the phase position Δφ, with which a high-frequency signal S₂ is emitted, is adjusted in such a manner that the high-frequency signal S₂ from the second signal generator 10 is displaced relative to the high-frequency signal S₁ from the first signal generator 2.by one phase, which approximately corresponds to the phase displacement through the device under test 3 with the addition of a phase angle of 180°. As a result, the two signals at the inputs 7 and 11 of the signal combiner 8 are coherent but inverted relative to one another, that is to say, the signals arrive at the signal combiner 8 simultaneously with the same amplitude but with a phase offset of 180°. The signal components of the input signal S₁ of the device under test 3 are therefore almost completely suppressed in the output signal S_(A) from the device under test. If power components are present in the adjacent channels CH⁻², CH⁻¹, CH₁, CH₂, these are also present in the reference signal S₂ and are suppressed because of the 180° phase position and are therefore not registered by the signal analyser 12.

With a WCDMA signal, a broad-band, modulated multiple carrier signal is used. According to one further development of the invention, the phase position of the modulation signal can therefore also be adjusted in such a manner that a 180°-phase difference is also produced at the signal combiner 8 for the modulation signal. For this purpose, the modulation signal, with which a high-frequency signal S₂ from the second signal generator 10 is modulated, is adjusted in such a manner that the modulation signal of the second signal generator 10 is displaced relative to the modulation signal of the first signal generator 2 by one phase, which corresponds as accurately as possible to the modulation phase displacement, that is to say, the phase displacement of the modulation signal through the device under test 3 with the addition of 180°.

In this context, it is advantageous if the phase position of the high-frequency signal and of the modulation signal can also both be adjusted with the smallest possible step width. Preferably, the phase positions Δφ and Δφ_(m) can be adjusted with a step width of less than {fraction (1/50)}, by further preference less than {fraction (1/100)} of the period of a high-frequency signal and/or of the modulation signal. Accordingly, with a sufficiently small step width, a broadband signal suppression of more than 26 dB can be achieved. As a result, an increase in measurement dynamics can be achieved with reference to the analyser 12, which must be driven with a lower signal level.

FIG. 3 shows a second exemplary embodiment of the measuring device 1 according to the invention. Elements which have already be described in the context of FIG. 1 are labelled with identical reference numbers to avoid repetition of the description.

By way of distinction from the exemplary embodiment illustrated in FIG. 1, the output 5 of the device under test 3 in the exemplary embodiment shown in FIG. 3 is not connected via a damping element 6 to the first input 7 of the signal combiner 8, but the output 5 of the device under test 3 is terminated with a terminal resistance 20, and at the output 5 of the device under test 3, a directional coupler 21 is provided, which decouples the output signal with a given coupling factor. While in the case of the exemplary embodiment shown in FIG. 1, the damping factor of the damping element 6 should have approximately the same magnitude as the amplification factor of the device under test 3; with the exemplary embodiment shown in FIG. 3, the coupling factor of the coupler 21 should have approximately the same magnitude as the amplification factor of the device under test 3, so that the signal amplitudes A₁ and A₂ present at the inputs 7 and 11 are of approximately the same order of magnitude, and only a fine adjustment needs to be made with the adjustment device 14 for balancing the amplitude A.

The invention is not restricted to the exemplary embodiments illustrated. For example, measurements of n-port devices, with n−1 input ports are conceivable, wherein a different input signal is supplied to each input port. In this context, for every input port, in addition to the signal generator for the measured signal, a signal generator should be provided for the reference signal, and all reference signals of the reference signal generators should be supplied to the signal combiner 8, which should, in this case, also be designed as an n-port device. 

1. Method for measuring intermodulation distortions of a device under test (3) comprising the following procedural stages: indirect or direct supply of an initial high-frequency signal (S₁), which is generated by a first signal generator (2), to an input (4) of the device under test (3), and indirect or direct supply of an output signal (S_(A)) from an output (5) of the device under test (3) via a signal combiner (8) to a signal analyser (12), characterised by indirect or direct supply of a second high-frequency signal (S₂), which is generated by a second signal generator (10) synchronised with the first signal generator (2), via the signal combiner (8) to the signal analyser (12).
 2. Method according to claim 1, characterised in that the second high-frequency signal (S₂) is generated in such a manner that it is inversely coherent to the first high-frequency signal (S₁).
 3. Method according to claim 1 or 2, characterised in that, at the first signal generator (2) and/or at the second signal generator (10), a time delay (ΔT), with which the high-frequency signal (S₁; S₂) is emitted from the first and/or second signal generator (2; 10) respectively, is adjusted in such a manner that the high-frequency signal (S₂) from the second signal generator (10) is delayed by a time offset relative to the high-frequency signal (S₁) emitted from the first signal generator (2), which approximately corresponds to the group delay through the device under test (3).
 4. Method according to any one of claims 1 to 3, characterized in that, at the first signal generator (2) and/or at the second signal generator (10), a phase position (Δφ), with which the high-frequency signal (S₁; S₂) is emitted from the first and/or second signal generator (2; 10), is adjusted in such a manner that the high-frequency signal (S₂) from the second signal generator (10) is displaced relative to the high-frequency signal (S₁) emitted from the first signal generator (2) by a phase, which approximately corresponds to the phase displacement through the device under test (3) with the addition of 180°.
 5. Method according to any one of claims 1 to 4, characterised in that the high-frequency signal (S₁, S₂) of the first and second signal generator (2, 10) is modulated with a modulation signal and, at the first signal generator (2) and/or at the second signal generator (10), a phase position (Δφ_(m)), with which the high-frequency signal is modulated with the modulation signal from the first and/or the second signal generator (2; 10), is adjusted in such a manner that the modulation signal from the second signal generator (10) is displaced relative to the modulation signal from the first signal generator (2) by a phase, which approximately corresponds to the modulation phase displacement through the device under test (3) with the addition of 180°.
 6. Method according to any one of claims 1 to 5, characterised in that, at the first signal generator (2) and/or at the second signal generator (10), an amplitude (A), with which the high-frequency signal (S₁; S₂) is emitted from the first and/or the second signal generator (2; 10), is adjusted in such a manner that the amplitudes (A₁, A₂), with which the high-frequency signals (S₁, S₂) from the signal generators (2, 10) arrive at the signal combiner (8), are approximately equal.
 7. Method according to any one of claims 1 to 6, characterised in that the device under test (3) is an amplifier, that a damping element (6) is arranged between the device under test (3) and the signal combiner (8), and that the damping factor of the damping element (6) is dimensioned in such a manner that the signal level at the output of the damping element (6) approximately agrees with the signal level at the input (4) of the device under test (3).
 8. Method according to any one of claims 1 to 6, characterised in that the device under test (3) is an amplifier, of which the output (5) is terminated with a terminal resistance (20), and that between the device under test (3) and the terminal resistance (20), a coupler (21) is arranged, of which the coupling factor is dimensioned in such a manner that the signal level at the output of the coupler (21) approximately agrees with the signal level at the input (4) of the device under test (3).
 9. Measuring device (1) for measuring intermodulation distortions of a device under test (3) comprising a first signal generator (2), which generates a first high-frequency signal (S₁), which is supplied indirectly or directly to an input (4) of the device under test (3), a signal combiner (8), of which the first input (7) is indirectly or directly connected to the output (5) of the device under test (3), and a signal analyser (12), which is indirectly or directly connected to the output of the signal combiner (8), characterised by a second signal generator (10) synchronised with the first signal generator (2), which generates a second high-frequency signal (S₂), which is supplied indirectly or directly to a second input (11) of the signal combiner (8).
 10. Measuring device according to claim 9, characterized in that the second signal generator (10) generates the second high-frequency signal (S₂) in a manner inversely coherent to the first high-frequency signal (S₁).
 11. Measuring device according to claim 9 or 10 characterized in that, in the first signal generator (2) and/or in the second signal generator (10), a time delay (ΔT), with which the high-frequency signal (S₁; S₂) is emitted from the first and/or the second signal generator (2; 10) respectively, can be adjusted, wherein the time delay (ΔT) is adjusted in such a manner, that the high-frequency signal (S₂) from the second signal generator (10) is delayed relative to the high-frequency signal (S₁) emitted from the first signal generator (2) by a time offset, which approximately corresponds to the group delay through the device under test (3).
 12. Measuring device according to any one of claims 9 to 11, characterised in that, in the first signal generator (2) and/or in the second signal generator (10), a phase position (Δφ), with which the high-frequency signal (S₁; S₂) is emitted from the first and/or the second signal generator (2; 10) respectively, can be adjusted, wherein the phase position (Δφ) is adjusted in such a manner that the high-frequency signal (S₂) from the second signal generator (10) is displaced relative to the high-frequency signal (S₁) emitted from the first signal generator (2) by a phase, which approximately corresponds to the phase displacement through the device under test (3) with the addition of 180°.
 13. Measuring device according to claim 12, characterised in that, the step width, with which the phase position (Δφ) of a high-frequency signal (S₁; S₂) from the signal generator (10) and/or the signal generators (2, 10) can be adjusted, is less than {fraction (1/50)}, preferably less than {fraction (1/100)} of the period of the high-frequency signal (S₁; S₂).
 14. Measuring device according to any one of claims 9 to 13, characterised in that the high-frequency signal (S₁; S₂) from the first and second signal generator (2, 10) is modulated with a modulation signal, and in the first signal generator (2) and/or in the second signal generator (10), a phase position (Δφ_(m)), with which the high-frequency signal is modulated with the modulation signal from the first and/or second signal generator (2, 10), can be adjusted, wherein the phase position (Δφ_(m)) is adjusted in such a manner that the modulation signal of the second signal generator (10) is displaced relative to the modulation signal of the first signal generator (2) by a phase, which approximately corresponds to the modulation phase displacement through the device under test (3) with the addition of 180°.
 15. Measuring device according to claim 14, characterised in that the step width, with which the phase position (Δφ) of the modulation signal (S₁; S₂) from the signal generator (10) and/or from the signal generators (2, 10) can be adjusted, is less than {fraction (1/50)}, preferably less than {fraction (1/100)} of the period of the modulation signal.
 16. Measuring device according to any one of claims 9 to 15, characterised in that, in the first signal generator (2) and/or the second signal generator (10), an amplitude (A), with which the high-frequency signal is emitted from the first and/or second signal generator (2, 10), can be adjusted, wherein the amplitude (A) is adjusted in such a manner that the amplitudes (A₁, A₂), with which the high-frequency signals (S₁, S₂) from the signal generators (2, 10) arrive at the signal combiner (8), are approximately equal.
 17. Measuring device according to any one of claims 9 to 16, characterised in that the device under test (3) is an amplifier, that a damping element (6) is arranged between the device under test (3) and the signal combiner (8), and that the damping factor of the damping element (6) is dimensioned in such a manner that the signal level at the output of the damping element (6) approximately agrees with the signal level at the input (4) of the device under test.
 18. Measuring device according to any one of claims 9 to 16, characterised in that the device under test (3) is an amplifier, of which the output (5) is terminated with a terminal resistance (20), and that between the device under test (3) and the terminal resistance (20), a coupler (21) is arranged, of which the coupling factor is dimensioned in such a manner that the signal level at the output of the coupler (21) approximately agrees with the signal level at the input (4) of the device under test (3).
 19. Measuring device according to any one of claims 9 to 18, characterised in that the signal analyser (12) records the signal (A₁) from the first signal generator (2) arriving at the signal combiner (8) and drives the second signal generator (10) in such a manner that this emits the second high-frequency signal (S₂) in dependence upon the recorded signal.
 20. Measuring device according to any one of claims 9 to 18, characterised in that the signal analyser (12) records the signal (A₂) from the second signal generator (10) arriving at the signal combiner (8) and drives the first signal generator (2) in such a manner that this emits the first high-frequency signal (S₁) in dependence upon the recorded signal. 